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Igneous rocks

Basalt columns of the Giant's CausewayBasalt columns of the Giant's Causeway Igneous rocks are formed when magma, molten rock from beneath the Earth’s crust, rises, cools and solidifies. Intrusive igneous rock, such as granite, is formed when magma becomes solid in cracks or chambers beneath the ground. Extrusive igneous rock, such as basalt, is formed when the magma solidifies above the ground quite quickly. This happens when magma seeps out on to the ocean bed, or when runny lava spreads out from a volcano on land over a large area.

Volcanic dykes in the Wrangell Mountains, AlaskaVolcanic dykes in the Wrangell Mountains, Alaska

A close-up view of granite rock.A close-up view of granite rock.

Intrusive igneous rock

Magma bubbles up through the crust wherever one plate sinks down beneath another in subduction zones. Water from the wet sediments on the ocean floor makes the magma more fluid, so it can rise through cracks in the crust above. The magma seeps into the crustal rocks, sometimes collecting in large chambers.

Magma turning to solid rock undergroundMagma turning to solid rock undergroundThere it may stay, eventually cooling down slowly to become a mass of igneous rock. Magma that cools deep underground is called intrusive igneous rock. Granite is an example of intrusive igneous rock.

There are more than 700 types of igneous rock.

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